The instant invention relates to burners and methods for using the burners for industrial heating including oxy-fuel burners employed in the transport channels leaving a glass melting furnace and those employed in small specialty glass melters.
Air-fuel glass conditioning burners, such as the burners used in refining or in the forehearth, do not use preheated air. Much of the energy supplied to the conditioning operation from the air-fuel burners is used to heat the nitrogen in the air. Efficiency improvement achievable by switching from air-fuel to oxy-fuel burners is at least 60%. Typical oxy-fuel glass conditioning burners are designed with high fuel velocities for rapid mixing of fuel and oxygen at the oxygen and fuel gas outlets. These burners generate flames that diverge widely as the distance from the nozzle increases. The burner block opening required to accommodate these flames must be divergent as well, providing the opportunity for volatile material to make its way into the block channel and deposit on the nozzle tips. Small nozzle tip openings are easily clogged from these deposits, causing glass contamination with molten metal from the damaged burner and unfavorable glass temperature distributions in the conditioning area that lead to glass quality issues. This nozzle clogging problem is not easy to identify before glass quality issues arise, since there is often a lack of a clear view of the flames in the glass conditioning and forehearth areas. Furthermore, partial clogging of the nozzle can misdirect the oxy-fuel flame and lead to burner block damage or direct flame impingement on the glass surface, causing glass contamination or volatilization of the high boiling components of the glass.
In the case of the distribution canals (e.g., within the forehearth) which transport the molten substance from the melting furnace to the installations downstream, such as shaping machines in particular, the burner or burners supply the energy necessary such that on entering the downstream (shaping) installation, the molten substance has homogeneous properties such as temperature, viscosity, among other properties, that are suited to its treatment in the downstream installation. The distribution canal may in particular be equipped with burners aimed at compensating for heat losses through the walls of the canals so as to reduce the heterogeneity, notably in terms of the temperature of the molten substance in the transverse section of the distribution canals. The burners of the distribution canals are then, as a general rule, low power burners the flame of which is limited to a zone near the walls of the canal.
When employing a flame in a distribution canal which extends beyond the zone near the walls of the canal and which therefore also heats the molten material in the central zone of the canal, there is substantial risk of reboiling the molten material in this zone and, therefore, of having an end product that exhibits inhomogeneities and corresponding defects.
The width of the molten material distribution channels may be small creating a challenge in maintaining a homogeneous temperature distribution while firing the burners at a firing rate sufficient to maintain the optimal glass temperature. For example, fiberglass forehearths are very narrow, often less than 18 inches wide which can present a challenge of supplying sufficient thermal energy to the system without flame impingement on the opposite wall.
Other applications for burners in glass melting operations may utilize medium to high firing rates and include specialty glass furnaces, such as pot furnaces and day tanks, or large capacity molten material distribution channels, canals and refiners. These applications require relatively high heat input per volume of furnace. This requires relatively high firing rate burners to provide sufficient heat input. At the same time, the distance between the furnace walls is relatively small. Air-fuel fired installations such as these achieve the required heat input within the confined space by firing the air-fuel burners in an arrangement where the air-fuel burner mounted on the opposite wall is directly opposed to the air-fuel burner mounted on the near wall. The high momentum of one of the opposed air-fuel flames effectively balances the high momentum of the other opposed air-fuel flame, protecting the opposite wall from overheating. This opposed air-fuel burner configuration creates a lot of turbulence in the combustion chamber which can create hot spots. Hot spots in the combustion chamber can also be generated if the flows to the opposed air-fuel burners are not well balanced or the burners are poorly aligned when installed. Even when this opposed installation method is employed, air-fuel burners are often unable to provide the desired amount of energy to the chamber, limiting glass temperature, compromising product quality and reducing production rates. Homogeneous glass temperature throughout the chamber is important to maintain product quality for these applications. It is desirable to have a means for providing the necessary thermal energy without generating hot spots on the refractory or within the molten material.
Burner design involves a variety of factors to consider, including fluid flow, mixing of gases and other considerations, particularly with respect to burner nozzles. Altering flow from nozzles having different aperture geometries to affect gas mixing has been studied. For example, a 1999 article by Gutmark, E. J. et al., entitled “Flow Control with Noncircular Jets” reports a trend for entrainment with different nozzle aperture geometries with air at room temperature. This data shows that the circle nozzle imparts the lowest amount of entrainment and the slot and zipper nozzle geometries promote higher entrainment. Higher entrainment for the nozzle aperture shape means more mixing between fuel and oxygen streams for a burner configured with fuel gas nozzle with the high entrainment design and an oxidant stream surrounding the fuel stream. Gollahalli, S. R. et al. (Combustion Sci. Technol. 1992, 86:1-6, 267 “Diffusion Flames of Gas Jets Issued from Circular and Elliptic Nozzles”), studied diffusion flames from circle and elliptical (aspect ratio major axis/minor axis 3:1) fuel nozzle apertures using nitrogen diluted propane fuel (Re 4740 for circle nozzle) and a low velocity concentric air stream. Gollahalli confirmed that Gutmark's results with the cold flow tests can be translated to air fuel propane flames. Gollahalli found enhanced mixing between fuel and air in flames produced from an elliptical fuel nozzle aperture compared with flames produced from a circle fuel nozzle aperture having the same open area. Gollahalli's tests were limited to the open air firing of air fuel flames from burners without precombustors. Gollahalli did not report any difference in the length of the flame produced from the two different nozzle geometries. In fact, the carbon monoxide concentration (a measure sometimes used to estimate flame length) measured at the mid range and furthest distance from the nozzle tip was characterized by the authors as ‘not significantly different’ for the two different nozzle geometries.
Another paper by Zaman, K. B. M. Q. Zaman, entitled “Axis Switching and Spreading of an Asymmetric Jet: The Role of Coherent Structure Dynamics”, discusses how flow develops with asymmetrical nozzles, mapping mean velocity contour slices as a function of distance from the nozzle. Zaman found that the spreading of the flows is enhanced for asymmetrical nozzles that have vortex generators. These vortex generators are essentially tabs machined on the walls of the slot that create a discontinuity along the wall. The authors also introduce the concept of axis inversion, where the flow can start spreading along the long axis of the slot and switch direction 180 degrees at some distance from the nozzle tip. The experiment where air flows through a slot nozzle shows that the flows spread in the direction of the asymmetry until at some distance from the nozzle the flow returns to a circular shape. The impact of the vortex generator is to disrupt the typical flow pattern developed by the smooth-walled nozzle aperture, leading to axis inversion (for delta tabs mounted on the short edge of the slot) or preventing the flows from reverting to a circular pattern, (stabilizing the spreading of the flows at larger distances from the nozzle). The articles from Zaman and Gutmark show flow behavior resisting the formation of the flat flame, where flat flow patterns either require complicated burner arrangements or only have flat flow patterns for a portion of the flow from the burner face. A nozzle with a slot shape but lacking a smooth edge (zipper) enhances mixing so much that the precombustor begins to overheat if the precombustor has a similar L/D as the precombustor used for the slot with the smooth surface.
Known combustion methods are described in U.S. Pat. No. 5,256,058; US Pub No. 2010/0310996; and U.S. Pat. No. 5,500,030; the disclosure of which is hereby incorporated by reference. The principle of the oxy-fuel burner disclosed in '058 is to delay mixing of the fuel and oxygen while confining the near nozzle portion of the oxy-fuel flame inside a precombustor to produce a highly luminous flame. This flame provides very efficient heat transfer in large furnaces and inhibits buildup and corrosion on the burner nozzles. One drawback of the delayed mixing approach is that the flames are too long to supply the needed energy to channels, canals and forehearths without overheating the glass in the center of a wide forehearth or overheating the refractory on the opposite wall of a narrow forehearth, canal or small specialty glass melting furnace. Another drawback is that the '058 flames do not are not capable of maintaining sufficient temperature homogeneity of the glass inside these glass conditioning structures, adversely affecting product quality.
One attempt at obtaining a flat flame is disclosed in Kobayashi, U.S. Patent Publication No. US2003/0015604, which is incorporated by reference in its entirety. Kobayashi discloses introducing a gas stream above and below the fuel gas stream to ‘flatten’ the flame. Kobayashi utilizes a complicated design and does not utilize fluid dynamics from the nozzle shape and burner operation to alter the flame geometry.
A forehearth burner is disclosed by International Publication No. WO2011/154285, which is incorporated by reference in its entirety. In the WO2011/154285 Publication, an oxy-fuel burner is disclosed for use with a forehearth of a glass melting furnace. However, the design disclosed has limited ability to alter the flame geometry and provide uniformity in heating.
An oxy-fuel burner arrangement, a combustion system, and method for enriching combustion of a combustible fuel with oxygen that is efficient and reduces or eliminates overheating of the burner components reduces or eliminates overheating of the refractory on the opposite wall of the furnace, while providing thermal energy in a manner that maintains a homogeneous temperature throughout the transport channel of molten material would be desirable in the art.